Magnet generators including a magnet rotor whose field system is configured by using a permanent magnet, and a stator made by winding a generation coil around an armature core having magnetic pole parts facing magnetic poles of the magnet rotor are widely used as generators attached to internal combustion engines. While magnet generators are AC generators, most electric components provided to machines having internal combustion engines mounted thereon are DC loads. Accordingly, when configuring a power supply device for supplying power to various electric components by using such a generator as a power source, a circuit for converting the output voltage of the generator into DC voltage is necessary.
When a device having an internal combustion engine mounted thereon is not provided with a battery, it is necessary to drive electric component loads that need to be constantly driven to cause the engine to operate, such as a fuel pump of a fuel injection device for supplying the internal combustion engine with fuel and an ignition device for igniting the engine, by using a power supply device using a generator driven by the internal combustion engine as a power source. In order to improve startability of the internal combustion engine in such a case, not only it is necessary to increase the voltage generated by the generator while the engine is rotating at extremely low speed and thereby make the rotation speed at which the supply of power from the power source to the loads is started as low as possible, it is also necessary to cause a necessary and sufficient level of voltage for driving the loads to be generated also while the engine is rotating at high speed.
Meanwhile, when a device having an internal combustion engine mounted thereon is provided with a battery, a power supply device using a generator driven by the engine as a power source is used to charge the battery. Also in such a power supply device for an internal combustion engine, it is necessary to cause a large output voltage to be generated while the engine is rotating at low speed, without sacrificing the output voltage while the engine is rotating at high speed.
In order to increase generator output voltage while an engine is rotating at extremely low speed, it is necessary to increase the number of turns of the generation coil. However, when increasing the number of turns of the generation coil, the inductance of the generation coil increases, and in addition, the resistance of the coil increases. Due to this, a decrease in the voltage generated while the engine is rotating at high speed cannot be avoided. Thus, the demand for an increase in voltage generated while the engine is rotating at low speed and the demand for an increase in voltage generated while the engine is rotating at high speed are mutually conflicting. It is therefore difficult to configure a power supply device satisfying both these demands merely by providing a generator driven by an engine and a circuit for rectifying output from the generator.
In view of this, as disclosed in Patent Document 1, there has been proposed a power supply device configured to perform chopper control for periodically interrupting a current flowing through a specific generation coil provided in a generator (a control of repeating an operation of causing a short-circuit current or a current similar thereto to flow through the generation coil and an operation of interrupting the current) while the engine is rotating at low speed, whereby boosted voltage is induced in the generation coil.
In the power supply device disclosed in Patent Document 1, a hybrid bridge circuit in which the upper sides of the bridge are configured by using diodes and the lower sides of the bridge are configured by using MOSFETs is used as a circuit converting an AC output voltage from the generation coil into a DC voltage. While the engine is rotating at low speed, chopper control for periodically interrupting the current flowing through the generation coil is performed by concurrently switching on and off the MOSFETs of both lower sides of the bridge, whereby a boosted voltage is induced in the generation coil. This boosted voltage is converted into DC voltage through rectification at a rectification circuit formed by parasitic diodes of the MOSFETs and the diodes of the upper sides of the bridge. By adopting this configuration, it is possible to increase the voltage induced in the generation coil while the engine is rotating at low speed and lower the rotation speed at which driving of loads is started. In addition, output from the generation coil while rotation speed is low can be increased without increasing the number of turns of the generation coil, whereby the voltage generated by the generation coil while the engine is rotating at high speed is prevented from decreasing.